Abstract

Proton delivery to the prosthetic group is a crucial step to sustain the activity of nitrite reductase. An iron N-confused porphyrin (NCP) complex, which is capable of relaying protons from the outer pyrrolic nitrogen (Nout–H) of the inverted pyrrole ring to the axial coordinated ligand, has been demonstrated to facilitate facile nitrite reduction. Time-dependent FTIR studies on the reaction between [FeII(HCTPPMe)Br] (1) and a nitrite anion revealed a two-step process involving conversion of the starting complex 1 to an {Fe(NO)}7 intermediate, [Fe(CTPPMe)(NO)] (5), before the detection of [Fe(CTPPCH2)(NO)] (3), an {Fe(NO)}6 end product. Moreover, spectroscopic data confirm that Nout–H on the NCP core is indispensable to the proceeding of the nitrite reduction reaction. Mass spectra have detected the coordination of a nitrite to the iron center while DFT theoretical calculations suggest that subsequent intramolecular proton transfer to a nitro group to form [Fe(CTPPMe)(HNO2)] (6a) evokes a homolytic N–OH bond fission on axial nitrous acid due to an enhanced π-back-bonding to produce an {Fe(NO)}7 intermediate and to release a hydroxyl radical. The subsequent oxidation of an {Fe(NO)}7 intermediate by the hydroxyl radical gave the final product, {Fe(NO)}6 [Fe(CTPPCH2)(NO)] (3). This study illustrates a proton assisted small molecule activation on the iron N-confused porphyrin coordination sphere and provides complemental insights into the mechanism of enzymatic nitrite reduction reactions.

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